1. Field of the Invention
The present invention relates to a method for making a glass or ceramic article, and more particularly to a method for making such an article by casting from a gellable suspension of oxide particles.
The concept of preparing ceramic or glass articles from suspensions of oxide particles is used in basic form in the field of slip casting. In that application, slips or slurries of oxide particles, typically greater than one micron in size, are cast, dried and fired to form amorphous glass or crystalline ceramic products. It is difficult, however, to prepare void-free and defect-free products by slip casting due to the relatively large sizes of the oxide particles employed.
Pure oxide particles can be produced by synthetic processes such as the vapor phase oxidation of volatile metallic or metalloid compounds, and articles fabricated from such pure oxides can have unique physical properties. U.S. Pat. No. 2,272,342 to Hyde describes the manufacture of pure fused silica products from SiCl.sub.4 by this process, and similar processes are presently used to produce the pure, highly transparent glasses used to fabricate low-loss optical waveguides for telecommunications applications.
Present techniques for shaping products from oxide particles typically involve the direct deposition of the particles from the vapor phase onto a removable substrate, followed by sintering to a unitary mass. This technique limits the configuration of the product, and other means for configuring products from particulate oxides have been sought. However, pure oxide particles produced by vapor phase oxidation are of sub-micron size, typically 0.01-0.5 microns in diameter in unagglomerated form, and are accordingly quite fluffy and difficult to handle.
U.S. Pat. Nos. 4,200,445 and 4,042,361 to Bihuniak et al. illustrate one approach toward solving this problem. In that method, aqueous suspensions of the oxides, termed fumed oxides because of the vapor phase method used for their production, are prepared, cast, dried and sintered to form a densified feed material which can be milled and shaped by slip casting. Glass resulting from this process is typically not of optical quality.
Published European patent application EP No. 0084438 describes an improved shaping method wherein products of arbitrary shape can be produced by the casting of non-aqueous colloidal oxide suspensions. In accordance with that method, submicron-sized oxide particles, e.g. fused silica particles, are first dispersed in a non-aqueous vehicle to form a stable fluid suspension, and the suspension is then formed, as by casting, into a product configuration and caused to gel in that configuration by the addition of a gelling agent. Thereafter, the gelled suspension is dried by removal of the vehicle, and sintered by heating to provide a unitary glass or ceramic product, e.g., of fused silica, having the same configuration (except for drying and sintering shrinkage) as that of the original casting.
It has been customary, in the preparation of suspensions of submicron oxide particles in accordance with the aforementioned process, to subject the particulate oxides to a heat treatment prior to dispersion in the non-aqueous vehicle in order to remove surface hydroxyl groups and adsorbed water commonly present on the surfaces of the particles. Further, the treated oxides are generally stored and dispersed under anhydrous conditions to avoid recontamination with water. These measures are employed because residual hydroxyl groups or water molecules on the particles were found to cause high viscosity and uncontrolled flocculation or gelling of the suspended oxides in the vehicles, particularly when it was desired to prepare suspensions of relatively high solids content.
The need to heat treat the oxides to remove surface hydroxyl groups and water, thereafter to store the oxides in dry form, and to disperse the oxides under anhydrous conditions, is considered disadvantageous because of the added cost of the heat treatment and special storage and handling. Hence a process wherein these steps could be avoided could be of considerable practical benefit.
It is therefore a principal object of the present invention to provide a process for manufacturing glass and ceramic articles from submicron-sized oxide particles which avoids the inconvenience of special handling and the need for a high temperature drying treatment.
Other objects and advantages of the invention will become apparent from the following description.